My husband and I are in are mid-30s. We prefer a table for 8. It seems to us that the table for 10 is too big...it's sometimes harder to really get to know the others at the table. We have also had good luck with 6 but our preference is 8. Hope that helps.
Ditto on the hearing problems depending on the ship and the layout of the dinning room. Was on a Princess cruise from Vancouver to Mexico one year at a table for 8 and could barely carry on a conversation because of the noise level. We have always enjoyed large tables but our best cruise was at a table for 4 where we had the most delightful senor couple as our tablemates. Also was seated one cruise at a table for 10 where the other 8 were traveling together, felt like odd man out.
Generally I agree to the table for 8, but on our last 7 day cruise we had a large round table for 10 that 11 sat around (we were an adult family group). I was surprised that we could all talk very comfortably, even my Mom and husband who have hearing problems in these situations.
We prefer a table for 4 or 6 but no more. It is difficult to hear conversations with the commotion around you at a larger table. Of course the advantage of a larger table is that you are with more people and a greater chance of meeting someone your own age and that you have something in common with. We have been on 11 cruises and have been seated at large and small tables and still prefer the smaller.